In the past, the sensing of direction of movement of a body and particularly a vehicle such as a rocket has been achieved either by reference to some internal standard such as a compass or gyroscope or by receiving external radio signals or the like from which the direction of movement can be determined. In the past, determining the speed of movement of an aircraft, rocket or the like has been based for example on sensing the air speed of the vehicle given that orientation of the vehicle relative to the direction of movement of the vehicle is known. That is, in the past, aircraft and rockets generally have a well defined "front" or "nose" pointed in the general direction of movement and that orientation has been implicity assumed in speed sensing systems.
It has been proposed to provide a defence against self-guided target seeking missiles by means of a decoy rocket which is launched upon detecting an approaching missile. It is proposed that the decoy rocket slowly move away from the target, which may be a ship, while at the same time emitting strong signals intended to be detected by the approaching missile so that the missile will guide itself towards the decoy rocket rather than the target ship. The air data unit for collecting data used to control the flight path of such a decoy rocket is desirably self-contained, i.e. there will be no direct control from the launch site. The air data unit will be operable to gather data such as the height, movement direction and speed of the rocket, for the determination of the flight path of the rocket and the subsequent flight control operations.
The prior devices for sensing the direction and speed of movement of aircraft or rockets and briefly outlined above are believed to be unsuitable for this kind of decoy rocket for several reasons. For example, gyroscopic direction sensing systems are expected to be too expensive. Since the decoy rocket is intended to be expandable, the component parts should therefore be relatively inexpensive. Movement direction sensing based on signals received from the launch site are unsuitable, because these signals may be subject to deliberate or accidential interference. Also, the decoy rocket is intended to be movable generally horizontally in any direction without that direction of movement being accurately known or even approximately known in relation to the configuration of the rocket itself. That is, the direction of movement of the rocket in relation to the configuration of the rocket will not be known in advance or will not be known for the remainder of the flight after launch, and conventional speed sensing devices based on air speed of a rocket are expected to be unsuitable.
The problem with measuring direction and/or speed of movement of the decoy rocket relates to the configuration and nature of movement of the rocket. For example, it is expected that the decoy rocket will be generally cylindrical and may be about seven feet long and about six inches in diameter. The rocket will be launched from a tube and the slope of the launching tube initially will determine the trajectory of the rocket, although the axis of the cylindrical body is expected to be generally vertical. At operational height, the rocket is expected to have a zero or small vertical speed so that it can hover or rise slowly and also move generally horizontally to attract the approaching missile from its target. For the flight control unit of the decoy rocket to effectively operate the direction of movement of the rocket must be known at least approximately and it will be seen that this direction determination may be difficult with a tubular rocket that can move in any horizontal direction generally at right angles to the upright axis of the cylindrical body.